This invention relates to plural-mode and multiple-cavity microwave filters and, more particularly, to a dual-mode microwave filter with large entrance and exit ports in lieu of input and output coupling irises for increased bandwidth and minimum losses.
Microwave bandpass filters find considerable use in microwave communication systems, as well as in radar, and signal processing circuits. Such filters are constructed to have specific passband characteristics for the transmission of a signal having prescribed bandwidth in a specific portion of the electromagnetic spectrum, while preventing the transmission of microwave signals lying outside of the passband. In the construction of such filters, it is highly desirable in many applications to provide a filter characteristic which is substantially free of frequency dispersion throughout the passband, thereby to ensure that higher and lower frequency components of a signal are equally attenuated by the filter so as to preserve the signal waveform. Preferably, such attenuation is no more than a small fraction of a decibel.
Desirable filter transmission characteristics have been obtained by providing the filters with a multiple cavity construction as is disclosed., by way of example, in U.S. Pat. Nos. 4,028,651 of Leetmaa and 4,251,787 of Young et al. The individual cavities may be separated by irises having cross-slotted apertures which enable the propagation of electromagnetic energy by waves in orthogonal modes through the filter.
A problem exists in that, upon enlargement of the passband beyond the usual range of passbands, presently available filters introduce a greater insertion loss and a greater dispersion than are desired.